Fix JsonWebKey potential null pointer dereference

In extractKeysFromJsonWebKeySet, if mJsonObjects size
is zero, mJsonObjects[0] will result in a null pointer
dereference.

Test: sts-tradefed
  sts-tradefed run sts-engbuild-no-spl-lock -m StsHostTestCases --test android.security.sts.Bug_176496483#testPocBug_176496483

Test: push to device with target_hwasan-userdebug build
  adb shell /data/local/tmp/Bug-17649648364

Bug: 176496483
Bug: 175955261
Change-Id: Ib70f1c6f9abb8f2628abb229fc389b83bf6e7552